Many recipes ask for a batter to rest before cooking, and most give no reason. Several distinct processes run during that wait, and they affect the finished item measurably.
Flour hydrates slowly
Flour particles absorb liquid over minutes rather than instantly, and a freshly mixed batter still contains dry starch at its centres.
Resting allows that absorption to complete, producing a batter of even consistency throughout rather than one that keeps thickening unpredictably while it is being cooked.
This is why a rested batter frequently appears thicker than it did at the moment of mixing, even though no liquid has been lost and nothing has been added to it.
Gluten relaxes after mixing
Mixing develops gluten strands and leaves them under tension, which makes a batter or dough resist spreading and contract when handled.
During a rest, enzymes and time together allow those strands to slacken and rearrange, so the batter flows more readily off the spoon and the finished item comes out noticeably tenderer.
For thin preparations such as crepes, that relaxation is the difference between a batter that spreads evenly and one that fights the pan.
Air bubbles redistribute or escape
Vigorous mixing incorporates large irregular air pockets, which produce uneven texture and visible holes in a delicate item.
Standing lets the largest of those rise and escape while smaller bubbles remain distributed, giving a finer and more even crumb.
The opposite applies to batters leavened chemically at the last moment, where waiting wastes the gas that was supposed to do the lifting.
Fermented batters are doing something else entirely
A rest of hours or overnight in a warm room is not hydration but fermentation, with microorganisms consuming sugars and producing acids and gas.
That changes flavour substantially, adds sourness, and produces the lift that makes fermented rice and lentil batters rise without any added leavening.
Temperature governs the pace, which is why the same batter is ready in hours in a warm kitchen and needs a full night in a cold one.
Resting has a limit and a cost
Beyond a point, hydration is complete and continued standing only allows separation, with heavier particles settling and liquid rising above them.
A brief stir before cooking recombines them, though vigorous re-mixing undoes the gluten relaxation that the rest achieved.
Batters containing raw egg and dairy also need refrigeration for long rests, which slows every process and lengthens the time required accordingly.